Spotted seals hear well in and out of water

Two spotted seals, named Amak and Tumu, were orphaned as pups and raised at the Long Marine Laboratory at the University of California, Santa Cruz. Hearing tests revealed that the animals hear frequencies that may mean they are susceptible to the effects of anthropogenic noise

Spotted seals live along the continental shelf in the northern parts of the Pacific, from Alaska to the Sea of Japan. The Arctic hold dangers for the seals: There are polar bears and brown bears on land, and sharks and orcas at sea. On top of that, climate change is destabilizing the region. But the seals appear to be hanging on.

Another threat for many marine creatures is sound — anthropogenic noise created by boats, oil and gas exploration and other human sources. These noises often occur at low frequencies. Spotted seals can hear in this frequency range, researchers report February 26 in the Journal of Experimental Biology, which may cause the animals stress or injury.

Jillian Sills of the University of California, Santa Cruz and colleagues tested the animal’s range of hearing by performing hearing tests on two young male spotted seals, Amak and Tunu, housed at the Long Marine Laboratory. The seals were orphaned as pups in the Arctic and were raised at the lab. The hearing tests took place in a pool of seawater and an above-ground acoustic chamber. Each seal was trained to touch a target when it heard a sound.

Their range of hearing was on par with that of harbor seals — a species with a range that overlaps that of the spotted seals — and better than that of other Arctic seals, the researchers report.

On land, the seals could hear as well as cats or dogs; they could detect four octaves of sound, from 0.6 to 11 kilohertz. Underwater, their range was seven octaves, from about 0.3 to 56 kilohertz, similar to porpoises and dolphins, though the seals’ range started at a much lower frequency. That ability to hear such low-frequency sounds suggests “they may be more vulnerable to the effects of anthropogenic noise,” the researchers write.

That spotted seals have good hearing both on land and sea is not all that surprising. Though the animals are well adapted to the water and can dive at least 300 meters in pursuit of a meal, they also spend weeks sitting out on ice floes or on land for pupping and molting seasons. They need good hearing to survive both environments. And living so far north means that there may be little light for much of the year, making them even more dependent on sound to warn them of predators and lead them to meals.